docs: def _compute_focal_depth_term(self, C, hypo_depth):
          """
          Compute fourth term in equation 1, p. 901.
          """
          # p. 901. "(i.e, depth is capped at 125 km)".
          focal_depth = hypo_depth
          if focal_depth > 125.0:
              focal_depth = 125.0
  
          # p. 902. "We used the value of 15 km for the
          # depth coefficient hc ...".
          hc = 15.0
  
          # p. 901. "When h is larger than hc, the depth terms takes
          # effect ...". The next sentence specifies h>=hc.
          return float(focal_depth >= hc) * C['e'] * (focal_depth - hc)